This is great, thank you so much for your patient support @BlueAnanas
For the record, here is the working command:
sudo -u mattermost /opt/gitlab/embedded/bin/mattermost -config="/var/opt/gitlab/mattermost/config.json" -permanent_delete_team=true -team_name="38fxueu1t3fobcdsyqk5hig49c"
and then replying with YES (all uppercase) to the two questions.